In known devices of this kind, rotary motion from an electric or pneumatic motor is transformed into advance movement of the spindle while it continues to be driven in rotation. The advance mechanism is generally constituted by a nut, with the spindle being given a left-hand thread for this purpose.
A system of gearwheels drives the screw or nut in rotation and gives rise to a speed difference between the speed of rotation of the rotating drive system and the speed of rotation of the screw. This speed difference is predetermined by the particular gearwheels selected and enables the spindle to be advanced or retracted by engaging or disengaging the advance gearwheels.
Such advance mechanisms are commonly found in the aviation industry (usually in association with pneumatic motors). Mechanical advance devices are generally associated with an abutment mounted on the spindle, either at its leading end or at its trailing end.
In the configuration of the invention, the abutment is situated at the trailing end of the machine. The abutment is located very specifically at a distance on the spindle that is preadjusted as a function of the stroke that is desired for the spindle. At the end of the stroke, the abutment comes into contact with a stationary or moving element of the machine that serves as a reference, such as a feeler bushing, the body of the machine, etc. By reaction against the internal mechanism, this contact causes the spindle to return automatically. This implies that for a machine with stroke adjustment at the rear of the machine, the space occupied by the machine is equal to the length of the stroke plus the length of the abutment.
Unfortunately, this can be incompatible with the space available for performing specific machining operations, in particular in corners, and in the aviation industry in the vicinity of the junction between the wing structure and the fuselage. The rear end of the spindle can come into contact with the part that is to be machined. As can be seen in FIG. 1, some present machines are unsuitable for performing such machining.
As shown diagrammatically in FIG. 1, if the spindle needs to have a stroke C for performing the desired work, this means that the space occupied by the rear portion is equal to E (sum of the stroke C+length of the abutment). The total space occupied A is equal to E plus the length of the body of the machine plus its forwardly-projecting barrel. This total space requirement is sometimes incompatible with the space available.